/Engine/Source/Runtime/Engine/Classes/Engine/Engine.h
Looks up the GUID of a package on disk. The package must NOT be in the autodownload cache. This may require loading the header of the package in question and is therefore slow.
static [FGuid](API\Runtime\Core\Misc\FGuid) GetPackageGuid ( [FName](API\Runtime\Core\UObject\FName) PackageName, bool bForPIE )